In recent years, there has been proposed a microwave-assist magnetic recording head in which a spin torque oscillator is provided as a microwave oscillator and a microwave magnetic field is applied to the magnetic recording layer of a magnetic disk from the spin torque oscillator when recording. For the spin torque oscillator used for microwave-assist recording, there are proposed an asymmetrical type which combines a perpendicular film and a soft magnetic layer, and a symmetrical type which oscillates in optical mode by combining two equivalent soft magnetism layers.
However, some symmetrical spin torque oscillators may be asymmetrical because of variation in processing during manufacture. Depending on such variation, the oscillating efficiency of the spin torque oscillator may decrease.